expensive at the dealer, brackets are around 25 quid, then you need the plastic shoots that sit in the tops of the brackets, they are 20 quid or so, and the headrests themselves are very expensive, about 55 each and i dont think that includes the covers (i would need top check).
can get part numbers and exact prices if you want
front headrests can be used in the rear brakets ONLY if u modify them, the legs need to be bent straight which is pretty difficult to master (genuine rear headrests have shorter straight legs)
your best bet if your on a budget is to try somewhere like stevens auto dismantlers for a second hand set up...(they go for quite alot second hand on places like Ebay when they come up)
not that hard to fit, did mine in ten mins but you had to take your seat to bits (very easy) and cut the fabric a little (again no biggie)
